A form of numerically controlled machining is one in which there is an area bounded by a closed curve, wherein surface cutting is applied to a portion (convex portion) projecting from an area on the outer side of the closed curve.
Such an area cutting (surface cutting) method includes the following steps:
(a) inputting data specifying a closed curve CCL of an area AR shown in FIG. 7, cutting direction (direction of arrow A) along cutting path PT.sub.i (i=1, 2, . . . ), shift direction (direction of arrow B) in which a tool TL is shifted by a predetermined amount of cut-in whenever surface cutting along the cutting path PT.sub.i ends, and cut-in amount P;
(b) generating the cutting path PT.sub.i on the basis of the inputted data;
(c) performing cutting by moving the tool along the cutting path PT.sub.i from a cutting starting point P.sub.i to a cutting end point Q.sub.i on the generated cutting path PT.sub.i ;
(d) obtaining the next cutting path PT.sub.i+1, which results when the tool is shifted by the amount of cut-in P following the end of cutting along the preceding cutting path;
(e) thereafter performing cutting (unidirectional cutting) by moving the tool from point P.sub.i+1 to point Q.sub.i+1, in which point P.sub.i+1 is taken as the cutting starting point of cutting path PT.sub.i+1 and point Q.sub.i+1 is taken as the cutting end point of cutting path PT.sub.i+1, or performing cutting (back-and-forth cutting) by moving the tool from point Q.sub.i+1 to point P.sub.i+1, in which point Q.sub.i+1 is taken as the cutting starting point of cutting path PT.sub.i+1 and point P.sub.i+1 is taken as the cutting end point of cutting path PT.sub.i+1 ; and
(f) repeating the unidirectional or back-and-forth cutting operation from this point onward to surface-cut the area AR. When cutting must be carried out by an up-cut or down-cut method, which depends upon the workpiece, surface cutting is performed unidirectionally. If the up-cut/down-cut method need be taken into consideration, surface cutting is performed by back-and-forth cutting.
In the conventional surface cutting method, the cutting starting point and end point of each cutting path PTi (i=1, 2, . . . ) are set fairly remote from the area boundary line so as not to leave any portions uncut. This results in a long tool pass along each cutting path, namely in a lengthy period of time during which surface cutting is not carried out. Consequently, machining efficiency is poor.
Accordingly, an object of the present invention is to provide a surface cutting method through which tool pass can be shortened without leaving portions uncut.
Another object of the present invention is to provide a surface cutting method through which a portion projecting from an area can be cut away in a reliable manner.